This invention relates to x-ray lithography in general and more particularly to an improved apparatus for generating x-rays for lithography and a method of using that apparatus.
With the need for better resolution in the lithography used in manufacturing microcircuits, new processes have been investigated. X-ray lithography has been proposed as a solution to the resolution problem. For example, see the article published in Electronics Letters, February 24, 1972, Volume 8, Number 4, at page 103 by D. L. Spears and Henry I. Smith of the Lincoln Laboratory at the Massachusetts Institute of Technology. The use of x-rays permits shorter wavelengths to be used. For example, the wavelength for the x-rays of interest is approximately 10 angstroms as compared to a wavelength of 4,000 Angstroms in ultraviolet.
The most commonly used x-ray target for lithography has been aluminum. The aluminum K-line is at about 8 angstroms which has been found to be a good wavelength for x-ray lithography. Basically, wavelengths between 4 and 14 angstroms are usable. However, using an aluminum target results in numerous disadvantages. Aluminum is subject to stress cracking and has a fairly low melting point. For these reasons, the amount of power which can be put into the system is limited if the aluminum is not to crack or melt. Because of these various factors, exposure time when using an aluminum target is relatively long. Consequently, the lifetime of an aluminum x-ray tube which operated at high power to get short exposure times is severely limited. For a typical design using an aluminum target, see the IBM Research Report entitled "High Brightness Ring Cathode Rotating Anode Source for X-ray Lithography" by G. A. Wardly et al. The use of a rotating anode to permit higher powers, which in itself was previously known, is described. Even with such a target, the maximum power is limited.
Traditional x-ray tubes, which have been used for medical, dental and similar purposes, utilize a high melting point target such as tungsten. However, tungsten has not previously been used for x-ray lithography because of the short wavelength radiation which it exhibits in the K-region; less than half an angstrom. Such a wavelength is not absorbed into the resist used on the microcircuit and thus cannot be used for x-ray lithography. Spears et al., in the aforementioned paper, did suggest the possibility of a tungsten target utilizing the continuum radiation. However, the results were not particularly promising. As noted, they had exposure times from 5 to 50 hours. With the mask structure used, the broad spectral distribution gave rise to an exposure contrast between the opaque and transparent regions of less than 3:1.
It is well recognized that, of prime importance in x-ray lithography, in addition to the need for good resolution, is the ability to process a large number of circuits in a short time. This dictates a short exposure time. In order to get a short exposure time generally requires increased power. To get high power requires a high melting point source.
Another approach taken has been to use a palladium or rhodium target and to dope the resist which is used to cause it to better absorb the radiation which is still relatively short at about 4 angstroms. Although this improves the processing time, it requires the (use by a doped) resist. In this method, it is not an increase of power which gives the shorter exposure time, but the response of the doped resist to the radiation.
Thus, the need for an improved apparatus for x-ray lithography and a method of using that apparatus quickly, efficiently and accurately to carry out a photo lithographic process, particularly for use in constructing microcircuits, becomes evident.